How to limit concurrency when using actors in Scala?

Question

I'm coming from Java, where I'd submit Runnables to an ExecutorService backed by a thread pool. It's very clear in Java how to set limits to the size of the thread pool.

I'm interested in using Scala actors, but I'm unclear on how to limit concurrency.

Let's just say, hypothetically, that I'm creating a web service which accepts "jobs". A job is submitted with POST requests, and I want my service to enqueue the job then immediately return 202 Accepted — i.e. the jobs are handled asynchronously.

If I'm using actors to process the jobs in the queue, how can I limit the number of simultaneous jobs that are processed?

I can think of a few different ways to approach this; I'm wondering if there's a community best practice, or at least, some clearly established approaches that are somewhat standard in the Scala world.

One approach I've thought of is having a single coordinator actor which would manage the job queue and the job-processing actors; I suppose it could use a simple int field to track how many jobs are currently being processed. I'm sure there'd be some gotchyas with that approach, however, such as making sure to track when an error occurs so as to decrement the number. That's why I'm wondering if Scala already provides a simpler or more encapsulated approach to this.

BTW I tried to ask this question a while ago but I asked it badly.

Thanks!

Answer 1

I'd really encourage you to have a look at Akka, an alternative Actor implementation for Scala.

http://www.akkasource.org

Akka already has a JAX-RS[1] integration and you could use that in concert with a LoadBalancer[2] to throttle how many actions can be done in parallell:

[1] http://doc.akkasource.org/rest [2] http://github.com/jboner/akka/blob/master/akka-patterns/src/main/scala/Patterns.scala

Answer 2

You can override the system properties actors.maxPoolSize and actors.corePoolSize which limit the size of the actor thread pool and then throw as many jobs at the pool as your actors can handle. Why do you think you need to throttle your reactions?